The present invention relates to a catalyst for purifying the exhaust gases from diesel engines which contain one or more zeolites and at least one platinum group metal. In a further aspect, the present invention relates to a method of using the catalysts as described herein for the purification of diesel engine exhaust.
The exhaust gases from diesel engines contain carbon monoxide, unburnt hydrocarbons, nitrogen oxides and particles of soot as air pollutants. The unburnt hydrocarbons include paraffins, olefins, aldehydes and aromatic compounds. In comparison to the exhaust gases from gasoline engines, diesel exhaust gases contain a substantially higher proportion of long-chain paraffins which are difficult to oxidize. In addition, diesel exhaust gases are substantially colder than the exhaust gases from gasoline engines and contain oxygen at a concentration between 3 and 10 volume percent.
The high oxygen concentration relates to the fact that diesel engines are operated with a large air/fuel ratio (kilograms of air to kilograms of fuel) of more than 18. Gasoline engines, in contrast, operate with an air/fuel ratio of 14.6, which enables stoichiometric combustion of hydrocarbons. The exhaust gases from gasoline engines therefore contain virtually no oxygen.
When operated under part-load, the exhaust gas temperature in a diesel engine is in the range 100 to 250.degree. C. and achieves a maximum temperature of 500 to 650.degree. C. only when operated under full load. In contrast, the exhaust gas temperature in a gasoline engine is between 400 and 450.degree. C. under part-load and can rise to 1000.degree. C. under full load.
The particles of soot in diesel exhaust gases consist of hydrocarbon cores with volatile organic components (VOC =volatile organic compounds) adsorbed thereon as well as adsorbed sulphates which are produced during combustion in a diesel engine as a result of the sulphur content of diesel fuel.
Due to the special properties of diesel exhaust gases, tailor-made exhaust gas purification systems have been developed for purifying them.
DE 39 40 758 A1 describes a catalyst for the oxidative purification of exhaust gases from diesel engines with high conversion rates for hydrocarbons and carbon monoxides at low temperatures and an inhibited oxidizing effect toward nitrogen oxides and sulphur dioxide. The active component in the catalyst consists of platinum, palladium, rhodium and/or iridium placed in contact with vanadium or an oxidic vanadium compound. The active component is deposited into finely divided aluminum oxide, titanium oxide, silicon oxide, zeolite or mixtures of these. The catalyst is applied in the form of a coating onto channels which allow the free passage of gases in a honeycomb shaped support made of ceramic or metal. The light-off temperatures T.sub.50% of this catalyst for carbon monoxide and hydrocarbons are in the range of 210 to 275.degree. C. (The light-off temperatures T.sub.50% are the temperatures of the exhaust gas at which just 50% of the pollutants are converted into harmless components.) At 350.degree. C. the catalyst exhibits good conversion rates for carbon monoxide and hydrocarbons. The catalyst allows nitrogen oxides to pass through virtually unchanged. Sulphur dioxide is oxidized to sulphur trioxide to only a very small extent. As a result of the dimininished oxidizing effect toward sulphur dioxide, this catalyst also leads to lower particle emissions than other oxidizing catalysts since less sulphate is available for adsorption onto the soot cores in the exhaust gases.
The problem of particle emission is being reduced by the planned introduction of low-sulphur diesel fuels, so the catalyst in DE 39 40 758 A1 will become less important.
EP 0 427 970 A2 describes a catalyst for reducing the amount of nitrogen oxides in an oxidizing exhaust gas with an air/fuel ratio of 22. The catalyst contains at least one zeolite with a molar ratio SiO.sub.2 /Al.sub.2 O.sub.3 of more than 10 and pore diameters of 0.5 to 1 nm. Platinum groups metals are deposited on the zeolites, wherein, for each platinum group metal, a minimum ratio by weight of metal to zeolite should not be undershot if good conversion rates for nitrogen oxides are still to be obtained even after aging of the catalyst.
DE 44 35 073 A1 describes a catalyst which contains a mixture of at least two zeolites with different pore diameters and also cerium oxide loaded with palladium. The mixture of zeolites is used to adsorb the differently sized hydrocarbon molecules in the exhaust gas during the cold-start phase.
Palladium and cerium oxide are used to convert the adsorbed hydrocarbons into harmless constituents.
An object of the present invention is to provided an improved catalyst, as compared with the prior art, for purifying the exhaust gases from diesel engines, which is capable of oxidizing in particular long-chain paraffins which are difficult to oxidize in the exhaust gas at temperatures below 200.degree. C., and simultaneously of reducing nitrogen oxides, despite the high oxygen content of the diesel exhaust gas.